The use of the BPH.4 solvent extraction system in the separation of tantalum from other refractory metals is under study. ACKNOWLEDGMENT
We acknowledge the laboratory assistance of J. H. Walker and the assistance of J. C. Sullivan of Argonne National Laboratories in some of the calculations involved in this work. The assignment of computer time and assistance by the staff of the Oklahoma State University Computer Center, also, is acknowledged.
( 7 ) Kaimal, V. R. M., Shome, S. C., Anal. Chim. Acta 31, 268-71 (1964). (8) Marcus, Y., Chem. Rev. 63, 139-70 (1963). (9) Milner, G. W. C., Wood, A. J., At. Energy Res. Estab. (Gt. Brit.) Rept. C I R 895. 1952. (lOf Moore, F. L., ANAL. CHEM. 35, 1032-4 (1963). (11) Morrison, G. H., Freiser, Henry, Zbid., 36,93R-l16R (1964). (12) Moshier, R . W., “Analytical Chemistry of Niobium and Tantalum,’] p. 88, Macmillan, New York, 1964. (13) Moshier, R . W., Schwarberg, J. E., ANAL.CHEM.,29,947-51 (1957). (14) Mvasoedov, B. F., Palshin, E. S., Palei; P. 8 . , Zh. Analat. Kham. 19, 105-lO(1964); C.A. 60, 1 2 6 4 6 ~ ( 1 9 6 4 ) . (15) Ni, Che-Ming, Chu, Chung-Fen,
Liang, Shu-Chuan, Hua Hsueh Hsueh Pa0 30, 290-5 (1964); C.A. 61, 11309d
LITERATURE CITED
( 1 ) Bantvsh. A. N.. Knvazev. D. A.. Levin4 0:V.,Zh,’ ,Veo;gan. Khim. 9; 2142-7 (1964); C.A. 6l,12931d(1964). ( 2 ) Belcher, R., Leonard, 31. A., West, T. S.. J . Chem. Soc. 1959,3577. ( 3 ) Goldstein, Gerald, ANAL. CHEM.36, 243-4 (1964). ( 4 ) Griffin, F. L., “An Introduction to Mathematical Analysis,” p. 471, Houghton Mifflin, Boston, 1936. ( 5 ) Healy, T. V., Solvent Extraction I - ,
Chemistry Symposium, U. S. Atomic Energy Commission, Gatlinburg, Tenn., October 23-26, 1962. Paper No. 5. ( 6 ) Ingri, N., Sillen, L. G., Arkiv Kemi, 23,97-121(1964) (Eng.).
(1964). (16) Xicolson, L. S., Varga, L. P., South-
west Regional Meeting, A.C.S., Dallas, Texas, Dec. 6-8, 1962. (17) Powell, R. H., Menis, Oscar, ANAL. CHEM.30,1546 (1958). (18) Pulley, P. G., Ph.D. Thesis, Department of Mathematics, Oklahoma State University, Stillwater, Okla.,
.Taniinrv. 196.5. - _..__ (19) Ryd&rg,-J., Sullivan, J. C., Acta. Chem. Scand. 13,2057 (1959). (20) Spitsyn, 5’. I., Radiokhimiya 6 , 120-4 (1964); C.A. 6 1 , 3 7 4 6 ~(1964). (21) Stevenson, P. C., Hicks, H. G., ANAL.CHEM.25, 1517 (1953).
(22) Sullivan, J. C., Rydberg, J., Miller, W. F., Acta Chem. Scand. 13, 2023 ( 19.59). (23) Tomioka, Hideo, Bunseki Kagaku 12, 271-9 (1963); C.A. 59. 5 7 6 8 ~ (1963). (24) Varga, L. P., Freund, Harry, J . Phys. Chem. 66,21, 187 (1962). (25) Varaa, L. P., Hume, D. N., Inoro. Chem. 2, 201 (1963). (26) Varga, L. P., Hume, D. N., Ibzd., 3 , 77 (1964). (27) Varga, L. P., Volz, UT.B., paper \ - - - - I -
presented at the Kinth A.C.S. Tetrasectional Meeting, Tulsa, Okla., March
16, 1963. (28) Varga, L. P., Zumwalt, W. E., Wyatt, W. G., Abstracts, p . 34-0, 148th Meeting, A.C.S., Chic’ago, Ill., AUK.31-Sept. 4, 1964. (29) Werning, J. R . et al., Ind. Eng. Chem. 46,644 (1954). ( 3 0 ) White, J. C., Ross, W. J., “Separa-
tions by Solvent Extraction with Tri-noctylphosphine Oxide,” C . S . At. Energy Comm., ,YAS-SS 3102, Feb. 8,
1961. (31) Yamamura, S. S., Wade, M . A., Sikes, J. A., ANAL.CHEM. 34, 1308 (1962).
RECEIVEDFebruary 2, 1965. Accepted April 22, 1965. Work supported, in part, by the Oklahoma State University Research Foundation and by the summer research participation program of the National Science Foundation.
Cation Exchange Separation of Uranium from Bismuth in Hydrochloric Acid-Isopropanol Medium J O H A N N KORKISCH and S. S. AHLUWALIA Analytical Institute, University o f Vienna, IX. Wahringerstrasse 38, Vienna, Austria
A method for the simple and rapid separation of small quantities of uranium from a large excess of bismuth is described. By using a medium isopropanol and consisting of 90% 10% 6N hydrochloric acid, uranium (VI) is completely retained on a small column of the strongly-acid cation exchange resin Dowex 50, whereas bismuth as the anionic chloride complex passes into the effluent unadsorbed. For the elution of uranium, 4 to 12N hydrochloric acid is employed. This method can b e used for the routine determination of uranium in bismuth containing amounts of uranium ranging from a few micrograms to milligrams. Because several other metal ions such as vanadium, molybdenum, zinc, cadmium, indium, etc., show a behavior similar to that of bismuth, their ready separation from uranium can also b e effected.
B
with most methods used for the micro determination of uranium, a separation of these two elements usually has to be ECAUSE BISMUTH IKTERFERES
carried out. I n the analysis of bismuth-uranium alloys containing a high percentage of bismuth, a method would be of great advantage which enables the smaller constituent, in this case uranium, to be adsorbed on the resin preferentially. I n this connection a n ion exchange method appears particularly attractive because of the possibility of using remote handling techniques of “hot” samples. Banerjee and Heyn (1) have shown that microgram amounts of uranium in the range of 50 to 100 pg. can be separated from bismuth (50 to 200 mg.) a t p H 1.0 to 1.5, by passing the sulfate solution through a column of the anion exchange resin Dowex 1, X10 in the sulfate form.* During this operation uranium as anionic sulfate complex is retained by the resin while bismuth passes through. One disadvantage of this method is that the volume of sorption solution and the column dimensions must also be increased when the amount of bismuth or uranium is increased, so that the operation is lengthened accordingly. Furthermore, hy-
drolysis of bismuth sulfate (bismuth forms only a very weak, if any, anionic sulfate complex) takes place if the pH is not adjusted precisely to its optimum region. Even then hydrolysis invariably occurs if the sample adjusted to this p H is not passed through the column within 2 hours after its preparation. All these disadvantages can be avoided by using hydrochloric acid solutions in which bismuth forms very strong anionic chloride complexes (especially a t l o a concentrations of this acid). Thus from a lh’ hydrochloric acid solution bismuth is strongly retained by Dowex 1 while uranium under the same condition passes into the effluent unadsorbed (4, 6). Using this separation principle it is possible to separate very large amounts of bismuth from uranium only if rather large resin columns are used for the quantitative retention of bismuth. When employing mixtures of miscible organic solvents with water containing hydrochloric acid, very similar results are obtained ( 8 ) ; bismuth is in all cases much stronger adsorbed than uranium. VOL. 37, NO. 8, JULY 1965
1009
According to investigations by Faris and Buchanan (2) the adsorption behavior of uranium and bismuth toward strongly basic anion exchange resins in aqueous nitric acid solutions of various acidities is quite similar, with bismuth showing slightly higher adsorption at all nitric acid concentrations. From organic solvent mixtures containing nitric acid the adsorption of bismuth is much higher than from pure aqueous solution. A separation principle based on this fact was described by Korkisch and Tera (IO). For the simultaneous adsorption of bismuth, uranium, and thorium 96% n-propanol-4% 5N nitric acid was used, and then uranium was eluted with 80% methan01-20% 5 N nitric acid. After the removal of thorium with 80% methanol-20yG 6 N hydrochloric acid, bismuth was eluted from the Dowex 1 resin column employing 1 N nitric acid. From all these results it is seen that anion exchange in sulfuric, hydrochloric, or nitric acid solutions does not offer the ideal solution for the problem of separating small amounts of uranium from great quantities of bismuth. From a cation exchange selectivity scale presented by Strelow et al. (14) it is seen that uranium(V1) cannot be separated from bismuth in pure aqueous solutions 0.1 to 4N in nitric acid on the cation exchanger Bio Rad AG 50W-X8. For example in 0.5W acid the distribution coefficients of these two elements were 69 and 79, respectively. I n sulfuric acid medium bismuth is much more strongly adsorbed on the same resin than uranium. Recently Nelson et al. ( I d ) have shown that uranium and bismuth in hydrochloric acid solutions are adsorbed on the strongly acid cation exchange resin Dowex 50, X4. Because the distribution coefficients for uranium and bismuth in 0.5N hydrochloric acid were found 100 and 1, respectively, the separation of these two elements from one another would be possible using this medium from which uranium is adsorbed while bismuth passes through. A disadvantage of a method developed on these lines would be, however, that the adsorption of uranium as well as that of bismuth in the acidity region from 0 to 1N hydrochloric acid is very much dependent on the acid concentration-i.e., a small change in the acidity increases or decreases the adsorption of both metal ions considerably. (The Kd value of uranium in I N acid is only about 20.) According to Strelow ( I S ) precipitation of bismuth occurs when the acidity is less than 0.3N. Consequently the acidity would always have to be closely controlled to avoid breakthrough of the uranium when the acidity is increased or to prevent hydrolysis and precipitation cf bismuth when the acidity is lowered. Such small changes in acidity may occur, especially when 1010
ANALYTICAL CHEMISTRY
great amounts of bismuth chloride are to be dissolved in acid. Besides, the column dimensions have to be selected by taking these small acidity changes into consideration which means that much longer columns than theoretically necessary will have to be used. This effect of acidity would be much less of a nuisance if the Kd value of uranium was considerably higher. Because of the fact that distribution coefficients on cation exchangers as a rule are increased when replacing part of the aqueous phase by miscible organic solvents-e.g., acetone (3)-the present investigations were carried out to find a medium from which uranium is relatively strongly adsorbed while bismuth is not retained. Among all the media investigated a mixture consisting of 90% isopropanol-10% 6 N hydrochloric acid is ideal for the separation of small quantities of uranium from considerable amounts of bismuth. EXPERIMENTAL
Reagents. T h e resin used for the separation experiments and measurements of the distribution coefficients was Dowex 50, X8 (100-200 mesh; hydrogen form). Before transferring i t to the columns the resin was soaked in a n excess of the wash solution (see below). For the determination of the batch distribution coefficients ( 7 ) of uranium, bismuth, and other elements the air-dried form of the resin was employed. Standard solutions of uranium, bismuth, and several other elements were prepared by dissohing reagent grade chlorides in 6 N hydrochloric acid. The wash solution was prepared by mixing 100 ml. of 6N hydrochloric acid with 900 ml. of isopropanol. When the small volume change after mixing is disregarded this solution is 90 volume % in isopropanol and 10 volume % in 6 N hydrochloric acid. Also used were other chemically pure organic solvents such as methanol, ethanol, acetone, etc. Apparatus. I n all separation experiments columns of a length of 10 em. a n d a diameter of 1 cm. mere used. These columns were constructed from polyethylene tubing and always contained 2 grams of the resin when the procedure described below was followed. T h e lower tip of the columns had a diameter of 0.5 cm. to ensure t h a t the solution can pass through a t a reasonable flow rate. Procedure. SEPARATION OF URANIUM FROM BISMUTH. Pretreatment of Resin Bed. The resin was soaked in the wash solution and transferred to the column followed by a 25-ml. wash with the same solution. Sorption Step. To the sample of bismuth chloride (equivalent to not more than 1 gram of bismuth) containing the uranium, 2 ml. of 6Ar hydrochloric acid were added, and aftw complete dissolution of the salts the solution was diluted by the addition of
18 ml. of isopropanol. An aliquot of this solution containing not more than 2.5 mg. of uranium was passed through thecolumn a t a flow rate of 0.3 to 0.4 ml./minute. If only very little uranium is present the entire 20 ml. of sorption solution may be passed. Washing Step. After complete sorption the resin bed was treated with the wash solution until the iodide test for bismuth in the effluent was negative. This was usually the case after less than 50 ml. have passed, provided that the amount of bismuth present in the aliquot percolated through the column did not exceed 0.5 gram. Elution. Afterwards uranium was eluted by passing 75 ml. of 4 to 12~V hydrochloric acid through the resin bed. After evaporation of the eluate, uranium was determined fluorometrically (4, 6). A blank determination showed that the presence of organic matter from the resin did not interfere with the uranium determination. If a 10-ml. aliquot of the sorption solution is used, the whole process, including the pretreatment of the resin bed, evaporation of the uranium eluate, and the subsequent determination of uranium, can be completed within 8 hours. Because of the automatic nature of these processes, many samples can be treated simultaneously. If this procedure is to be applied to the analysis of bismuth-uranium alloys 1 gram of the sample is dissolved in nitric acid and the solution is evaporated to dryness on a water bath. The residue is then treated with concentrated hydrochloric acid and the solution is evaporated to dryness on the water bath. This step is repeated until all nitrate is removed. The chloride salts are then dissolved and the ion exchange operation is carried out as described above. RESULTS AND DISCUSSION
Prior to the development of the procedure outlined above, the cation exchange behavior of uranium in various organic solvent-water mixtures was investigated. These media consisted of 10% of 6 N hydrochloric acid and of 90% of water-organic solvent mixtures of a composition varying from 0 to 90% with respect to the organic constituent. Measurements of the batch distribution coefficients of uranium in these media gave the results shown in Table I. From these results it is seen that the Kd of uranium generally increases withan increase in the percentage of the organic solvent in the mixtures. Although this increase was more pronounced in analogous mixtures containing nitric and hydrobromic acid, respectively ( 5 ) , the hydrochloric acid-organic solvent mixtures are more suitable with respect to the separation of uranium from bismuth because of the great complexing strength of this acid towards bismuth. From Table I it is evident that especially the medium consisting of 90%
Table I.
f
Distribution Coefficients of Uranium in Various Organic Solvent-Water Mixtures Containing 10% 6N Hydrochloric Acid (Dowex 50) (5-mg. load per 1 gram of resin)
70Organic solvent Solvent Methanol Ethanol n-Propanol Isopropanol n-Butanol Isobutanol Acetone Tetrahydrofuran Methyl glycol Ethyl glycol Acetic acid
0
-------
\= :
*E
: I
0
0.P
0.4
0.6
0.8
I
I
1.0
I
J
I.¶
OVERALL ACIDITY (NORMALITY OF HCL)
Figure 1. Effect of acidity on distribution coefficients of uranium and bismuth I. Uranium in 90% iropropanol-lO% hydrochloric acid (Dowex 50, X8) 11. Uranium in 90% water-1 0% hydrochloric acid Dowex 50, X4(12); Bio-Rad AG 50W, XB (14) Ill. Uranium In 90% water-1 0% hydrochlorlc acid (Dowex 50, X 8 ) IV. Bismuth in 90% isopropanol or water-1 0% hydrochloric acid (Dowex 50, X8) .Precipitation of bismuth
...
90
80
60
40
20
0
85 315 266 500 141 193 6 1 2 30 14
207 180 159 400 67 150 4 11 61
144 137 102 224 1
67 70 41 120 11 9 96 76 51 95 54
45 46 61 60 37 34 54 77 45 48 42
40 40 40 40 40 40 40 40 40 40 40
22
a Kd value for uranium of only about 60 was earlier found by Korkisch and Hazan (9) using Dowex 1. To investigate the 90% isopropanolhydrochloric acid system more closely, the Kd values of uranium and bismuth were measured as a function of the acidity. The results of these studies are shown in the Figure 1. Into this figure showing the variation of the log Kd of uranium and bismuth in 90% isopropanol the results of measurements have been included which were obtained by Nelson et al. (12), Strelow ( I d ) , and the present authors using 90% water in place of the alcohol under identical conditions of acidity. A comparison of the curves obtained in the 9oa/o isopropanol media with those measured in the 90% water mix10% aqueous hytures (90% water drochloric acid) shows that for the separation of uranium from bismuth the isopropanol media of low acidity are much more suitable. This is because of the much larger separation factors and higher Kd values of uranium in these mixtures. For example, the separation factors ( K d uranium/Kd bismuth) in the 90% isopropanol and 90% water media of the same over-all acidity of 0.6N hydrochloric acid (90% 10% 6N hydrochloric acid) solvent are 500 and 40, respectively. From Table I it is seen that a decrease in the concentration of isopropanol causes a decrease of the Kd value of uranium. To investigate also the effect of variations of the percentage of isopropanol on the Kd value of bismuth, analogous experiments were carried out using this metal ion. The results of these measurements showed that the Kd value of bismuth is about one in all cases; that is, it is independent of the concentration of isopropanol. Because the Kd of bismuth is also' not dependent on the over-all acidity of the isopropanol-hydrochloric acid mixtures (see Figure 1) its separability from uranium is only a function of the Kd value of uranium which is dependent
+
isopropanol-1070 6 N hydrochloric acid is best suited for this purpose as the Kd of uranium in this medium is the highest in all the hydrochloric acid media investigated. Consequently all further studies were carried out in this medium. The strong adsorption of uranium from this 90% isopropanol-lO~o 6N hydrochloric acid mixture cannot readily be explained because under the same experimental conditions uranium is also adsorbed strongly on the strongly basic anion exchange resin Dowex 1, X8 ( 7 ) with a Kd value of about 8000, suggesting strong anionic complex formation. A similar phenomenon in aqueous solutions containing high concentrations. of hydrochloric acid was observed by Kraus et al. (11) who found that gold (111), iron(III), and gallium(II1) are strongly adsorbed by the Dowex 50 cation exchange resin although these metal ions form strong anionic complexes under comparable conditions. If in the 90% isopropanol-10% 6h' hydrochloric acid medium the acid component is replaced by hydrobromic acid, a n extremely high distribution coefficient for uranium is obtained (6). This is, however, not surprising because
85
+
1 102 77 16 150 65
Table 11. Separation of Uranium from Bismuth in 90% Isopropanol-10% 6N Hydrochloric Acid Medium (2-gram column of Dowex 50)
5 25 25 50 125 250 250 1250 2500 2500
50 50 250 500 250 50 500 250 500 500
5 24.5 25.3 50 5 126.3 252 248 2 1225 2500 2550
0 -2 t-1.2 C1
+i +o
9 -0 7 -2 0 +2
on both concentration of isopropanol and hydrochloric acid. For the separation experiments (see procedure) an over-all acidity of 0.6N was selected because in this medium containing 90% isopropanol the Kd of uranium ( = 500) is suitably high, and small changes in acidity do not change the Kd value appreciably (see Figure I), This would be the case, however, if the separations were carried out a t over-all acidities ranging from 0.2 to 0.06X or lower and 0.9 to 1.2N, respectively. I n the first case much higher separation factors for uranium would exist but bismuth chloride hydrolyzes and precipitates a t these low acidities. In the acidity region from 0.9 to 1.2iV the Kd values of uranium are too low to be of practical importance with regard to capacity of the resin for uranium and speed of the sorption process. (The use of a much larger resin column would be necessary and consequently the performance of the separation process would be lengthened considerably.) Investigations concerning the breakthrough capacity of the resin, by using goy0 isopropanol-lOyo 6 N hydrochloric acid as the sorption solution and wash solution, showed that on a &gram column of the resin on which 50 mg. of uranium were adsorbed the first fluorometrically detectable trace of uranium VOL. 37, NO. 8, JULY 1 9 6 5
e
101 1
Table 111. Distribution Coefficients of IsoSeveral Metal Ions in 90% propanol-1 0% 6 N Hydrochloric Acid Medium (Dowex 50)
(5-mg. load per 1 gram of resin)
Metal ion
Distribution coefficient 250 450 6 20
Cd(I1) Cdi 11)
Ce(II1) Zr(1V) Th(1Y) v1 I v-v I
1 1
very high very high very high 5
(about 0.1 pg.) was found in 50 ml. of the effluent after 500 ml. of the wash solution had already passed through the resin bed. Because of this high breakthrough capacity, uranium in microgram and milligram amounts can readily be separated from bismuth even if this
metal ion is present in very great excess. Results of such separation experiments (carried out according to the recommended procedure) using variable amounts of uranium and bismuth are shown in Table I1 from which it is seen that within the limits of error of the fluorometric procedure of *2%, all separations are quantitative. For the elution of uranium, 4 to 12N hydrochloric acid is preferred over more dilute acid because it allows the rapid removal of uranium. By investigating the adsorption behavior of other metal ions under the conditions used for the separation of uranium from bismuth, the results recorded in Table I11 were obtained. From the values of the distribution coefficients of the various metal ions shown in Table I11 it is seen that all elements which are known to form strong anionic complexes in pure aqueous, and also mixed aqueous, organic solvents containing hydrochloric acid (7) have very low Kd values, except for cobalt which shows the same anomalous behavior as uranium. All these elements can be easily separated from uranium by using the working procedure, except Fe(II1) which is in the medium used, partly present as Fe(II), [formed by the reduction of Fe(II1) by isopropanol and the resin], and which is strongly retained by the resin. The result of an experiment in which 2.5
mg. of uranium plus 250 mg. of cadmium were used showed that quantitative separation is achieved as in the case of bismuth. LITERATURE CITED
( 1 ) Banerjee, G., Heyn, A. H. A., ANAL. CHEM. 30, 1795 (1958). (2) Faris, J. P., Buchanan, R. F., Rept. ANL-6811, July 1964. ( 3 ) Fritz, J. S., Abbink, J. E., ANAL.
CHEM.,in press.
( 4 ) Hazan, I., Korkisch, J., Arrhenius, G., 2. Analyt. Chem., in press. ( 5 ) Korkisch, J., Rept. to Z.A.E.A. and U . S . At. Energy Comm. under Contract 67/US (At(30-1)-2623),April 1965. ( 6 ) Korkisch, J., Hazan, I., ASAL.CHEM. 36,2464 (1964). ( 7 ) Korkisch, J., Hazan, I., Tahnta 11, 1157 (1964). (8) Korkisch, J., Hazan, I., unpublished
results.
19) Korkisch. J.. Hazan., I.., ANAL. CHEM.. ' 37, 707 (1965): (10) Korkisch, J., Tera, F., 2. Analyt. Chem. 186, 290 (1962). ( 1 1 ) Kraus, K. A , , hlichelson, I). C., Nelson. F.. J . Am. Chem. SOC.81. 3204 (1959).' ' (12) Nelson, F., Murase, T., Kraus, K . A., J . Chromatog. 13,503 (1964). (13) Strelow, F. W. E., ANAL.CHEM. 32, 1186 (1960). (14) Strelow, F. W. E., Rethemeyer, R., Bothma, C. J. C., Ibid., 37, 106 (1965).
RECEIVEDfor review February 23, 1965. Accepted March 29, 1965. Research sponsored by the International Atomic Energy Agency and the U. S. Atomic Energy Commission under Contract 67/US (AT(30-1)-2623).
Refractometric Column Monitoring in Ion Exchange Chromatography of Carboxylic Acids KAZUKO SHIMOMURA and HAROLD F. WALTON University of Colorado, Boulder, Colo.
A flowing differential refractometer has been used with a potentiometric recorder to monitor effluents in anion exchange elution chromatography. The characteristics and limitations of this monitoring system have been examined. Nitrate-borate solutions were used as eluents; elution volumes for 1 2 carboxylic acids are reported. Elution volumes are greater for dicarboxylic acids than monocarboxylic, and they are increased by the presence of hydroxyl groups and ethylenic double bonds.
A
hindrance to the use of ion exchange chromatography has been the difficulty of analyzing the column effluents. Continuous monitoring can be used if the substances absorb in the ultraviolet or visible region, PRACTICAL
10 1 2
ANALYTICAL CHEMISTRY
and sometimes simple chemical tests can be made on individual fractions. These tests can be automated by the Technicon AutoAnalyzer (4). Nevertheless there has been a great need for a simple, general method of column monitoring that would be comparable in convenience with the methods used in gas chromatography. Refractive index provides such a method. Equipment is available that permits continuous recording of very small differences in refractive index. I n column chromatography the difference in refractive index between the solution flowing into the column and that flowing out can be measured and recorded. Refractive index differences of the order and less can easily be detected. We have established the usefulness of this method by applying it to the elution
chromatography of a number of simple aliphatic carboxylic acids, using a strong base anion exchange resin and eluting solutions containing sodium nitrate, sodium borate, and boric acid. We repeated the work of Schenker and Rieman ( 5 ) on the separation of malic, tartaric, and citric acids, then went on to study the effect of eluent composition on elution volume, and measured elution volumes of 12 acids ranging in complexity from acetic to citric. EXPERIMENTAL
Refractometer. The instrument used was a flowing differential refractometer, Model R4, made by Waters Associates, Inc., Framingham, Mass. The construction of this instrument has been described (3). The sample solution and the reference solutions each flow through one half of a
